The present invention relates to a control system for a plant, and more particularly to a control system for a plant having a controller for controlling a plant using a controlled object model which is obtained by modeling the plant.
There has been known a sliding mode controller for controlling a plant according to a sliding mode control using a controlled object model which is obtained by modeling the plant (Japanese Patent Laid-open No. 2000-110636, for example). A control period of this sliding mode controller is set to a value which is identical to a sampling period of an input and an output of the plant for defining the controlled object model.
According to the sliding mode control, it is possible to specify a damping characteristic of a control deviation between a control target value and an output of a plant to be controlled. Such a control is referred to as a response-specifying control. Other than the sliding mode control, a back-stepping control is known as a response-specifying control.
If a plant to be controlled has nonlinear disturbance such as friction, then it is necessary to shorten a control period of the controller in order to improve controllability of the plant. Specifically, moving a movable controlled element at a shorter control period makes it possible to eliminate motion discontinuities due to the static friction. In addition, a time period required for the input to reflect a change in the target value, i.e., the dead time of the plant control becomes longer, as the control period becomes longer. Therefore, it is effective to reduce the control period also for the purpose of improving performance of the plant output to follow the target value.
However, if the control period of the controller is set to a smaller value for improving controllability and the sampling period for defining a controlled object model of the plant is set to a value which is equal to the control period, the problems described below may occur.
If a change rate of the plant output is low compared with the control period (i.e., the plant output changes a little in a control period), then model parameters representing the characteristics of the controlled object model are identified according to data sampled at a relatively short period compared with the change rate of the plant output. Therefore, an amount of change in the sampled data (a difference between the two adjacent sampled data) is small, and the model parameters identified according to the sampled data are unable to accurately represent the dynamic characteristics of the plant.
According to the response-specifying control such as a sliding mode control described above, a feedback input is determined based on a function which specifies a damping characteristic of the control deviation. If data sampled at intervals of a short period are used, then an amount of change in the control deviation (the difference between a control deviation calculated from sampled data in the preceding cycle and a control deviation calculated from sampled data in the present cycle) becomes small. Accordingly, a value calculated by the above function becomes near zero, which makes a sensitivity of the feedback process lower. As a result, the specified response may not be achieved in some cases.